Solid-state vs. Spray-drying synthesis for Mg-doped P2-Na0.67Fe0.5Mn0.5O2 as cathode material for Sodium-Ion Batteries

Abstract

Among the different cathodes studied for Sodium-Ion Batteries (SIBs), the P2-layered oxide structure has garnered significant attention for its electrochemical properties. However, several critical issues must still be addressed to enable large-scale commercialization, including the numerous phase transitions that the structure undergoes during cycling at high potentials (4 V) and the low Na content in the pristine material. In this work, starting from the promising and sustainable Na_0.67Mn_0.5Fe_0.5O₂, we developed new cathodes by partially replacing Fe with Mg. In addition to studying the role of Mg(II), a cation known for its stabilizing properties, we also evaluated the influence of two different synthesis methods on the structural and functional properties. In particular, spray-drying proved to be very promising compared to the conventional solid-state synthesis, as it leads to materials with morphology and microstructure more compatible with application as cathodes for batteries. The calculated Na diffusion coefficient (D_Na⁺), e.g., is more than two orders of magnitude higher for P2-Na_0.67Mn_0.5Fe_0.3Mg_0.2O₂ prepared by spray-drying than by the solid-state (10^-8 vs. 10^-10 cm² s^-1). In line with this result, the capacity retention after 100 cycles @1C is also significantly higher (72% vs. 81%). Compared with P2-Na_0.67Mn_0.5Fe_0.5O₂, Mg(II)-doping significantly improves the cathodic performances of the spray-dried materials, increasing capacity retention after 200 cycles at 1C from 39% to 69%. In conclusion, we confirmed that the choice of the correct synthesis route, combined with optimization of elemental composition, plays a crucial role in the development of highly performance materials for SIBs.